Plastic hose and tubing is used to transport a variety of different gases and liquids. The plastic tubing must be connected at either end using some type of fitting. A hard fitting such as a metal fitting is particularly useful and preferred. Generally these have a barbed tubular portion which slides into the hose or tubing and a coupling such as a threaded section or the like at the opposite end. The tubular portion is preferably barbed and is larger than the internal diameter of the tube. The barbed portion is forced into the end of the tube stretching it and is held by compression.
In many applications compression fitment is not adequate over the long term. Thermal cycles allow the tubing to expand. Also, internal pressures tend to act upon the tubing causing it to expand. Such barbed-style compression fittings rely on the elastic properties of the material to maintain sealing compressions. These designs are prone to leakage with temperature cycling as the plastic material relaxes.
There are spring-type clamps that can be installed after the assembly is made which cause compression pressure in a narrow area under the clamp. These require a secondary operation to install. Unless several of the clamps are applied, only a very small portion of the barb is actually affected.
Other types of fittings form a seal from the deformation of a metal sealing ring or ferrule compressed around the tubing by tightening a threaded nut. The sealing force relies on the elastic properties of the plastic material retaining compression when deformed. This fitting design is common and gives adequate performance. However if the assembly is exposed to cycling temperature or the plastic material relaxes and the sealing compression is reduced, leakage may occur. An exemplary such device is shown in FIG. 4. This device also employs a spiral spring to prevent the end of the tube from kinking. Such a spiral spring does not actually engage the plastic tubing holding it in position. The spring itself is used purely for prevention of kinking of the end of the hose as is commonly used.
Another such device is shown in U.S. Pat. No. 5,286,068 which actually uses a multi-component fitting to force the spring against the tubing itself and the tubing itself simply is marginally engaged by the spring itself. Other spring-type clamps are disclosed for example in Sweger, U.S. Pat. No. 3,941,254 and Assenheimer, U.S. Pat. No. 6,193,382 B1.
The present invention is premised upon the realization that an extremely secure tight connection can be formed between the end of a coupling and plastic tubing wherein the exterior of the plastic tubing is engaged by a coil spring. More particularly, such a coupling is formed by placing a coiled spring around a distal end of a plastic tube and inserting a barbed tubular member into the interior of the tube. The barbed tubular member causes the plastic tubing to expand causing the coil spring to likewise expand and at the same time apply compressive force against the exterior of the tube.
This coupling finds particular application in automotive environments particularly air brake tubing. However, it can be used in virtually any application in which a plastic tube is fitted with a hard or metal coupling. The particular metal coupling can be either a male or female coupling. Further it can be a threaded or bayonet-type fitment or other snap-on fitment or the like.
The objects and advantages of the present invention will be further appreciated in light of the following detailed description and drawings in which: